The present invention relates to a method and apparatus for neutralization of unwanted signals from a direct current, or more particularly, to a method for cancelling hum emanating from a DC power supply to a triode amplifier.
Amplifiers used for the amplification of radio frequency ("RF") signals commonly derive their power from a direct current ("DC") power supply. Frequently, such DC power supplies are energized by an alternating current ("AC") signal which is repeatedly switched at a hiqh frequency to provide both rectification and regulation of the power signal. Frequently, the power signal is not at a constant DC voltage but has both random and periodic variations in amplitude. The periodic variations, sometimes called "hum", often have the same frequency as the alternating current energizing the power supply.
If variations are present in the DC power signal supplying an RF amplifier, such variations are sensed by the amplifier and passed through, with or without amplification to the output signal from the amplifier. Hence, great care must be taken in eliminating such power supply variations.
It is known to remove hum from a DC power supply voltage by the use of a capacitor shunt across the power voltage, shunting unwanted AC signals to ground while permitting the desired DC signal to pass. However, such capacitors usually do not eliminate all the hum signal from the power supply voltage. Moreover, the use of a large and stable capacitor which could be utilized in a circuit for a power supply in a high voltage RF amplifier is expensive, both in terms of the cost of the capacitor and the utilization of space within an amplifier enclosure.
Various circuit arrangements for elimination of substantial variations and hum in a supply voltage have been proposed. For example, numerous rectifier circuits and filter circuits have been designed to reduce variations in the power supply voltage before the voltage is applied to an amplifier. Additionally, it is known to compensate for variations in supply voltage by introducing a portion of the supply voltage to the grid electrode of a conventional tube-type amplifier.
For example, the Shepard, Jr. U.S. Pat. No. 2,313,097 dated Mar. 9, 1943 discloses the cancellation of the unwanted hum signal by introducing on the screen grid a proportion of the supply voltage variation sufficient to provide an equal amplitude and opoosite polarity variation to the variation normally present in the output circuit of the amplifier. Such prior art systems for reducing hum, often require that the frequency of the variation in the power supply be known prior to design of the compensating circuit. This requirement stems from the fact that the compensating circuit often contains components which must be tuned to the hum frequency in order to isolate the hum signal. The usefulness of these types of compensating circuit is reduced if the power supply variation is of a random or unknown frequency.
Similarly, the Barnes U.S. Pat. No 3,042,877 dated July 3, 1962 discloses a hum compensator in which a supply voltage hum signal derived by a transformer is applied to the cathode of a conventional audio signal triode amplifier. Such prior art hum compensation systems commonly depend upon the positive transconductance of the electron tube being used as an amplifier. While this assumption is valid for most audio frequency amplifiers, similar amplifiers used for RF signals frequently exhibit an opposite reaction to the application of an RF signal. Therefore, many of the hum compensation techniques used for audio frequency amplifiers are not appropriate for RF signal amplifiers. Indeed, the use of audio amplifier compensation techniques may only serve to exacerbate the RF signal variations because the compensating signals would be applied to the amplifier in the opposite phase as is needed to compensate, resulting in reinforcement, not a cancellation, of the variation signal.
It is accordingly an object of the present invention to obviate these and other known problems in RF signal amplifier circuits and to provide a novel method and apparatus for reducing power supply induced variations in the output signal of an RF amplifier.
It is another object of the present invention to provide a novel method and apparatus for reducing power supply variations without the use of large and expensive capacitor filters.
It is yet another object of the present invention to provide a novel method and apparatus for eliminating hum in RF signal amplifiers.
These and many other objects and advantages of the present invention will be apparent to one skilled in the art from the claims and from the following detailed description when read in conjunction with the appended drawings.